Prepare the pastry up to 2 days ahead. You can also caramelise the onion and cook the bacon a couple of days ahead as well. Store in the refrigerator til needed.
In a food processor combine the flour, salt, and cold cubed butter, pulse to process until the mixture resembles crumbs. Through the chute of a food processor add ice cold water one tablespoon at a time and pulse until the mixture feels a bit heavier. It will not come together just yet.
Remove the mixture from the food processor and empty into a large bowl. Bring the dough together with two hands, don't handle it too much, just enough to form a shaggy ball. Wrap in plastic wrap, then press the dough down with the palm of you hand to form a disk. Chill in a refrigerator for 20 minutes.
Take the shortcrust from the refrigerator and let it come to room temperature for at least 20 minutes. It's impossible to roll out a well chilled disk of pie crust and if you try really hard it will just crack. Once it's pliable enough to work with, start rolling it out on floured surface into a circle 12 inch in diameter. Don't worry if it cracks, just pinch it together and continue on. Drape on your rolling pin and transfer the rolled out shortcrust into a 9 inch pie plate. Once again if it cracks or tears, just patch it up.
Once the pie plate is lined with the shortcrust, fold the overhanging edges under to form a thicker rim, then crimp it decoratively. Chill in the fridge for 20 minutes. Preheat the oven to 200C/400F.
Take the shortcrust out of the fridge, prick the crust with a fork all over, then line it with parchment paper and fill with baking beans. Blind bake at 200C/400F for 20 minutes, then remove the baking beans and the paper and bake for 5 minutes longer. It will probably shrink a bit, which is normal. It should look pale and partially baked.
While the shortcrust is baking cook the onion and bacon. To a frying pan add the bacon and cook until the fat is rendered and the bacon is crispy. Remove to a plate. To the same pan add the sliced onion, if the pan is too dry add a touch of oil, cook over low heat for 15-20 minutes until cooked all the way through and caramelised. (UK bacon is much leaner than in the US. If you are cooking in the US or Canada, discard most of the bacon fat but leave approximately 1 tbsp to cook the onion.) Allow the bacon to cool, then chop it.
Break up the blue cheese with a fork. If using other type of cheese, grate it. In a large bowl whisk the eggs, double or heavy cream and milk with salt and pepper.
It’s time to assemble the quiche. Turn the oven temperature to 160C/325F. Scatter half of the blue cheese over the base of the pastry, followed by the caramelised onions and chopped bacon, top with the remaining cheese and place the parsley sprigs directly on the cheese. Carefully pour the egg mixture over the filling trying not to disturb it too much.
Bake in the preheated oven for 35-40 minutes or until set. Do not overtake or it will taste dry and rubbery!